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# Solve by substitution or addition?

Update:

I’m having trouble understanding this. I have looked through my notes but I really need help. Relevance
• By addition 3x = 18 and so x = 6 and by substitution y = 3

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• Solving by adding the equations:

x + 3y = 15

2x -  3y =   3

–––––––––––

3x +  0y = 18

3x = 18

Divide both sides by 3

x = 6 <––––––

Substitute x = 6 into

x + 3y = 15

6 + 3y = 15

subtract 6 from both sides

3y =  9

divide both sides by 3

y = 3 <–––––

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x + 3y = 15

2x -3y = 3 add these two equations together.

3x + 0 = 18

x = 6; so that 3y = 15 - 6 = 9 and y = 3

x = 6; y = 3 ANS

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• By Substitution

x = 15 - 3y

Hence 2(15 - 3y) - 3y = 3

30 - 6y - 3y = 3

30 - 9y = 3

9y = 27

y = 3

Hence

x + 3(3) = 15

x + 9 = 15

x = 6

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